1. Field of the Invention
This invention relates to the protection and surface treatment and coating of metal substrates and to surface treatment and coating compositions employed as solutions or dispersions and to the method of using these compositions for the treatment of aluminum, aluminum alloys and coated aluminum substrates. The compositions coated on the metal surfaces provide improved corrosion resistance, adhesion of overlaying coatings e.g. paints, and maintains low electrical contact resistance in a corrosive environment. More specifically, this invention relates to a novel corrosion resistant composition and to the process of using the composition on aluminum, aluminum alloys and coated aluminum substrates which comprises an acidic aqueous solution containing effective amounts of at least one water soluble trivalent chromium salt, an alkali metal hexafluorozirconate, at least one water soluble thickener and water soluble surfactants.
2. Description of Prior Art
It is a current practice to improve the corrosion resistance and bonding of subsequent coatings to metal surfaces by initially coating or treating the metal surface with protective films. The coating enhances the corrosion resistance of the unpainted metal surface and prepares the surface for a finish coating such as paint. These conversion coatings are most often applied by the use of hexavalent chromium-containing solutions. While these coatings provide good corrosion resistance, attempts have been made to provide a more acceptable non-chromate derived coating because of the growing concern regarding the occupational, safety, health and environmental effects of hexavalent chromium. Hexavalent chromium is highly toxic and is a known carcinogen. Therefore, the solutions used to deposit these protective films and the films per se are toxic carcinogenic. Chromate films, however, provide outstanding paint adhesion and corrosion resistance and are easy to apply by various methods including immersion, spraying or by the wipe-on technique. Environmental laws and OSH regulations are forcing the military and commercial users to find other non-toxic non-chromate pretreatments. Moreover, the use of chromate conversion coatings is becoming more expensive as the regulations are being enforced and costs become prohibitive with the restrictions being imposed by the EPA. In addition, certain processes like spraying chromate conversion coatings are forbidden because of OSH, thereby forcing the use of less than optimum alternative methods.
More specifically, it is known that aqueous chromate solutions contain chemicals that partially dissolve the surface of the metal and form insoluble films known as chromate conversion coatings. These coatings are corrosion resistant and protect the metal from various elements which cause corrosion. Although the conversion coatings enhance corrosion resistance and improve the paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the solutions from which they are made and the presence of hexavalent chromium in the applied films. This is a serious problem from the standpoint of the operators handling the solution e.g. disposing the used chromate solution and the chromate-contaminated rinse water, and the coating systems contaminated with chromates. These problems, however, can be avoided by eliminating the hexavalent chromium from the process. However, this method is expensive and can be a major cost factor in the overall metal treating process. Therefore it is highly desirable to provide processes and protective coatings which are free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and paint bonding properties which are comparable to those imparted by conventional chromate-based conversion coatings. Of particular interest is the use of chromate conversion coatings on aluminum alloys e.g. the coating of large objects such as aircraft. It would be desirable to provide a protective coating for aluminum, its alloys and coated aluminum utilizing relatively non-toxic chemicals that could serve as an alternative to the toxic hexavalent chromium.
This invention relates to acidic aqueous trivalent chromium compositions i.e. solutions of trivalent chromium salts and to a process of preparing corrosion resistant coatings on aluminum, aluminum alloys and coated aluminum substrates which comprises treating or coating the aluminum substrates at ambient temperatures with effective amounts of the aqueous solution to form a corrosion resistant coating or film on the surface of the substrate. The acidic aqueous solutions of this invention comprise per liter of water, from about 3 to 22 grams of a water soluble basic trivalent chromium salt, and preferably from 3 to 12 grams of chromium salt e.g. 5-7 grams, about 1.5 to 11.5 grams, and preferably 3 to 9 grams e.g. 6-8 grams, of an alkali metal hexafluorozirconate, from about 0 to 10 grams e.g. from 0.5 to 2.0 grams of a water soluble thickener and from about 0 to 10 grams e.g. from 0.5 to 2.0 grams of a water soluble surfactant.
It is therefore an object of this invention to provide a solution comprising a trivalent chromium salt and an alkali metal hexafluorozirconate for preparing corrosion resistant substrates of aluminum, aluminum alloys and coated aluminum.
It is another object of this invention to provide an aqueous solution or dispersion for treating aluminum, aluminum alloys and coated aluminum which contain trivalent chromium salts to improve the corrosion resistant, paint adhesion and provide aluminum substrates with low electrical contact resistance in a corrosive environment.
It is still a further object of this invention to provide an aqueous solution comprising a water soluble trivalent chromium salt in combination with an alkali metal hexafluorozirconate, a water soluble thickener and a water soluble surfactant for preparing corrosion resistant coatings on aluminum and aluminum alloy substrates.
These and other objects will become apparent to those skilled in the art from a further and more detailed description of the invention as follows:
More specifically, this invention relates to an acidic aqueous solution of a trivalent chromium salt having a pH ranging from about 2-6 e.g. 4-5 and the use thereof for preparing corrosion resistant coatings or films on aluminum, aluminum alloys and coated aluminum substrates which comprises from about 3.0 to 22 grams per liter of solution and preferably about 3 to 12 grams per liter e.g. 5-7 grams of at least one water =soluble trivalent chromium salt, about 1.5 grams to 11.5 grams and preferably about 3 to 9 grams e.g. 6-8 grams per liter of solution of at least one alkali metal hexafluorozirconate, from about 0.0 to 10 grams per liter of the solution and preferably from about 0.5 to 2.0 grams per liter of at least one water soluble or dispersible thickener e.g. water soluble organic thickener and from about 0.0 to 10 grams per liter and preferably 0.5 to 2.0 grams per liter of the solution of at least one water soluble surfactant or wetting agent selected from the group consisting of nonionic, cationic and anionic surfactants. The trivalent chromium may be added as any water-soluble trivalent chromium compound, preferably as a trivalent chromium salt. Although the resultant coating is rinsed with water, it is desirable to use salts that provide anions that are not as corrosive as the chlorides which include anions selected from the group consisting of nitrates, sulphate, phosphate and acetates.
Specifically, in formulating the aqueous coatings of this invention, the chromium can be added conveniently to the solution in its water soluble form wherein the valence of the chromium is plus 3. The preferred chromium compounds are incorporated in the solution in the form of Cr2(SO4)3, (NH4)Cr(SO4)2 or KCr(SO4)2 and mixtures of these compounds. The aluminum surface is either pure aluminum, an aluminum alloy or coated aluminum substrate containing over 60% aluminum. The most preferred trivalent chromium concentration is within the range of about 5 to 7 grams per liter of the aqueous solution. It has been found that particularly good results are obtained economically when the chromium is present in this preferred range. The most preferred metal fluorozirconate addition to the solution ranges from about 6 to 8 grams per liter. The treatment or coating of the aluminum surface can be carried out at various temperatures including the temperature of the solution which ranges from ambient e.g. room temperature up to about 200xc2x0 F. Room temperature treatment is preferred in that this eliminates the necessity for heating equipment. The coating may be air dried by any of the methods known in the art, for example, oven drying, forced air drying, exposure to infra-red lamps, etc. For purposes of this invention, the terms aluminum and aluminum alloys include pretreated or coated substrates of aluminum and its alloys including a process known as ion vacuum deposition (IVD) where the aluminum alloy substrate is pretreated or coated e.g. with aluminum before the substrate is coated with the trivalent chromium salt solution of this invention.